A structure of the SOI type generally comprises at least one layer of insulator sandwiched between a thin upper layer and a base substrate, typically composed of silicon. “Thin layer” is taken to be a layer with a thickness typically in the range between 50 Å and a few micrometers, for example, around a hundred Angstroms, for example, 120 Å.
The layer of insulator can be a layer of oxide, such as SiO2, which is then referred to by the acronym “BOX,” for “buried oxide,” in other words oxide buried under the thin layer of silicon.
The fabrication of structures of the SOI type generally comprises the following steps:                formation of a layer of oxide on the surface of a substrate made of a semiconductor material such as silicon;        implantation of atomic or ionic species into the substrate, through the layer of oxide, so as to form a fragilization region within the substrate, and to bound a thin layer of semiconductor material between the layer of oxide and the fragilization region;        bonding of the substrate onto a second substrate; and        removal of the back part of the first substrate, by detachment along the fragilization region, so as to conserve only the structure of the SOI type.        
It is also possible to carry out the implantation of the atomic or ionic species into the substrate and to form the layer of insulator on the second substrate.
Nevertheless, implanting the species through the insulator offers certain advantages. This allows the implantation depth of the species to be made uniform, by notably reducing a phenomenon known as “channeling,” during which the implanted species along a main direction of the crystal lattice formed by the thin layer propagate to a greater depth than the mean depth reached by the implanted species.
Furthermore, as indicated hereinabove, the fragilization region defined by the implanted species bounds the thin layer to be detached from the substrate. Making the implantation depth of the species uniform, therefore, allows the thickness of the thin layer to be made more uniform, this thickness being subject to very tight controls because it strongly influences the quality of the SOI structure obtained.
However, in the latest SOI structures developed, called “UTBOX” for Ultra-Thin Buried Oxide, in which the thickness of the layer of oxide is less than around 50 nm (such a layer will be referred to as “thin oxide film”), the inventors have observed that the thickness uniformity of implantation of the species is degraded when the implanter used is of the batch implanter type.
In this type of implanter, a plurality of substrates to be implanted are positioned on as many supports disposed around a perimeter of a wheel rotating about its axis of symmetry, and exposed, in turn, to a flux of species maintained in a constant direction.
Surprisingly, the use of a batch implanter for implanting an UTBOX structure does not allow satisfactory results in terms of implantation depth to be obtained. Furthermore, in the case of an SOI structure of the UTBOX type, the specifications on the thickness uniformity of the thin layer of semiconductor material to be detached from the substrate are even more severe than for conventional SOI structures.
It is, therefore, necessary to develop, notably for SOI structures of the UTBOX type, novel solutions to allow an improved uniformity of the implantation depth of the ionic or atomic species in the substrate.